The present invention relates generally to systems for radio receivers operative to receive differentially-encoded, phase-modulated signals, and, more particularly, to a system, and associated method, for determining the absolute phase of the differentially-encoded, phase-modulated signals received by the radio receiver.
A communication system is operative to transmit information (referred to hereinbelow as an "information signal") between two or more locations, and includes a transmitter and a receiver interconnected by a transmission channel. Information is transmitted by the transmitter to the receiver upon the transmission channel. A radio communication system is a communication system in which the transmission channel comprises a radio-frequency channel wherein the radio-frequency channel is defined by a range of frequencies of the electromagnetic frequency spectrum.
The transmitter forming a portion of a radio communication system includes circuitry for converting the information signal which is to be transmitted into a form suitable for transmission thereof upon the radio-frequency channel. Such circuitry is referred to as modulation circuitry which performs a process referred to as modulation. In such a process, the information signal is impressed upon a radio-frequency electromagnetic wave wherein the radio-frequency electromagnetic wave is of a frequency within the range of frequencies defining the radio-frequency channel upon which the information signal is to be transmitted. The radio-frequency electromagnetic wave is commonly referred to as the "carrier signal", and the radio-frequency electromagnetic wave, once modulated by the information signal, is commonly referred to as the modulated signal.
Various modulation schemes are known for impressing the information signal upon the carrier signal to form the modulated signal.
One such modulation scheme is phase modulation in which the information signal is impressed upon the carrier signal in a manner to cause the phase of the carrier signal to be altered corresponding to the information content of the information signal. Phase changes of the modulated signal thereby form the information content of the modulated signal. Proper detection of the phase of the modulated signal permits recreation of the information signal.
A related modulation scheme is differential phase modulation in which differential phase changes of the modulated signal (i.e., phase differences between adjacent portions of the modulated signal) form the information content of the modulated signal. Proper detection of differential phase changes of the modulated signal permits recreation of the information signal.
Radio communication systems are advantageous in that no physical interconnection is required between the transmitter and the receiver; once the information signal is modulated to form the modulated signal, the modulated signal may be transmitted over large distances.
A cellular, communication system is one type of radio communication system. Radio telephones operative in such a cellular, communication system contain circuitry permitting simultaneous generation and reception of modulated signals, to permit thereby two-way communication between the radio telephones and remotely-located transceivers. These remotely-located transceivers, commonly referred to as "base stations", are physically connected to conventional telephonic networks to permit communication between a radio telephone and a fixed location of the conventional telephonic network.
A cellular, communication system is formed by positioning numerous base stations at spaced-apart locations throughout a geographical area. Each base station contains circuitry to receive modulated signals transmitted thereto by one, or many, radio telephones, and to transmit modulated signals to the one, or many, radio telephones. A frequency band (in the United States, extending between 800 MHz and 900 MHz) is allocated for radio telephone communication upon a cellular, communication system.
The positioning of each of the base stations forming the cellular, communication system is carefully selected to ensure that at least one base station is positioned to receive a modulated signal transmitted by a radio telephone positioned at any location throughout the geographical area.
Because of the spaced-apart nature of the positioning of the base stations, portions of the geographical area throughout which the base stations are located are associated with individual ones of the base stations. Portions of the geographical area proximate to each of the spaced-apart base stations define "cells" wherein the plurality of cells (each associated with a base station) together form the geographical area encompassed by the cellular, communication system. A radio telephone positioned within the boundaries of any of the cells of the cellular, communication system may transmit, and receive, modulated signals to, and from, at least one base station.
As the base stations and radio telephones of the cellular, communication system contain circuitry to permit continuous and uninterrupted communication between the radio telephone and a base station associated with a cell in which the radio telephone is positioned as the ratio telephone is moved between cells, communication upon a cellular, communication system is particularly advantageous by one operating a radio telephone when travelling in an automotive vehicle.
Increased popularity of communication upon a cellular, communication system has resulted, in some instances, in the full utilization of every available channel of the frequency band allocated for cellular, radio telephone communication. As a result, various ideas have been proposed to utilize more effectively the frequency band allocated for radio telephone communication. By more efficiently utilizing the frequency band allocated for radio telephone communication, the transmission capacity of an existing cellular, communication system may be increased.
One such proposal permits two or more radio telephones to share a single transmission channel. When the two or more radio telephones transmit or receive signals over a single transmission channel, the capacity of an existing cellular, communication system may be doubled. While the signals transmitted from, or to, the radio telephones which share the same transmission channel can not be simultaneously transmitted (simultaneous transmission would cause signal overlapping, thereby preventing signal detection of either of the signals), the signals can be transmitted in intermittent bursts. By encoding an information signal into discrete form (to form, e.g., a discrete, binary data stream) and modulating the discretely-encoded signal generated by such encoding process, the resultant modulated signal may be transmitted in intermittent bursts. Such modulated signals may be recreated by the receiver to determine thereby the information content of the transmitted signal.
A modulation technique suitable for modulating the discretely-encoded information signal upon a carrier signal is the aforementioned, differential, phase modulation technique. More particularly, a specific, differential modulation technique, a .pi./4 differential, phase-shift-keying (DQPSK) modulation technique has been selected as the standard modulation technique for cellular, communication systems of increased capacity in the United States.
Encoding of an information signal into a discrete binary data stream is also advantageous as noise introduced upon the modulated signal during transmission thereof upon the transmission channel may be more easily detected and removed when the information signal is comprised of a discrete binary data stream than when the information signal is comprised of a conventional, analog signal.
In a receiver operative to receive such a modulated signal, evaluation of the absolute phase of the signal is not required to recreate the information signal, but, rather, changes in phase of the modulated signal are evaluated to recreate the information signal.
Distortion occurring during transmission of a modulated signal comprised of a discretely-encoded, information signal (and modulated by the .pi./4 DQPSK modulation technique above-noted) may be removed by equalizer circuitry forming a portion of the receiver circuitry. The equalizer may, for instance, comprise a maximum likelihood sequence estimator (MLSE) such as that described in an article entitled "Adaptive Maximum-Likelihood Receiver For Carrier-Modulated Data-Transmission System", by Gottfried Ungerboeck in the IEEE Transaction On Communication, Volume COM-22, No. 5, May 1974.
The MLSE disclosed therein is comprised of a matched filter and a Viterbi equalizer. Both the matched filter and the Viterbi equalizer may be implemented by an algorithm embodied in processor circuitry.
A modulated signal received by the receiver is demodulated by the modulator circuitry, and then applied to the matched filter of the MLSE. The matched filter generates a filtered signal which is supplied to the Viterbi equalizer. The Viterbi equalizer is operative to correct for distortions of the signal occurring during transmission thereof upon a frequency channel.
Equalizers which, as mentioned hereinabove, form portions of radio receivers operative to receive digitally-modulated signals are operative responsive to determination of the absolute phase of the modulated signals transmitted thereto. Because the information content of the DQPSK signal is contained in the phase changes of the signal rather than the absolute phase of the signal, equalizer circuitry inefficiently, and sometimes ineffectively, corrects for the distortions to a DQPSK signal received by the receiver.
What is needed, therefore, is means for the determining the absolute phase value of a differentially-encoded, phase-modulated signal transmitted to a receiver.